:~# aptitude updateHit http://security.debian.org/debian-security stretch/updates InReleaseIgn http://ftp.de.debian.org/debian stretch InReleaseHit http://neurodebian.ovgu.de/debian data InReleaseHit http://ftp.de.debian.org/debian stretch-updates InReleaseHit http://neurodebian.ovgu.de/debian stretch InReleaseHit http://ftp.de.debian.org/debian stretch ReleaseW: http://security.debian.org/debian-security/dists/stretch/updates/InRelease: The key(s) in the keyring /etc/apt/trusted.gpg are ignored as the file is not readable by user '_apt' executing apt-key.W: http://neurodebian.ovgu.de/debian/dists/data/InRelease: The key(s) in the keyring /etc/apt/trusted.gpg are ignored as the file is not readable by user '_apt' executing apt-key.W: http://ftp.de.debian.org/debian/dists/stretch-updates/InRelease: The key(s) in the keyring /etc/apt/trusted.gpg are ignored as the file is not readable by user '_apt' executing apt-key.W: http://neurodebian.ovgu.de/debian/dists/stretch/InRelease: The key(s) in the keyring /etc/apt/trusted.gpg are ignored as the file is not readable by user '_apt' executing apt-key.W: http://ftp.de.debian.org/debian/dists/stretch/Release.gpg: The key(s) in the keyring /etc/apt/trusted.gpg are ignored as the file is not readable by user '_apt' executing apt-key.

Because I installed Psychtoolbox-3 via the Debian packages, I do not need the source and the key anymore and I think it is best to delete them both (as they are making problems right now anyway). apt-key list gives the following:

Just to make sure that I am not doing something worse (like deleting the wrong key) because of unknowing: I think the command to delete the key should be something like "apt-key del <keyname>" but I am not really sure whether it is correct and which part I have to add for the NeuroDebian key in the last line. Can I simply delete the file 'sources.list.d/neurodebian.sources.list' in '/etc/apt/sources.list.d/neurodebian.sources.list' to remove NeuroDebian source?

:~# aptitude updateHit http://security.debian.org/debian-security stretch/updates InRelease Ign http://ftp.de.debian.org/debian stretch InReleaseHit http://ftp.de.debian.org/debian stretch-updates InReleaseHit http://ftp.de.debian.org/debian stretch ReleaseW: http://security.debian.org/debian-security/dists/stretch/updates/InRelease: The key(s) in the keyring /etc/apt/trusted.gpg are ignored as the file is not readable by user '_apt' executing apt-key.W: http://ftp.de.debian.org/debian/dists/stretch-updates/InRelease: The key(s) in the keyring /etc/apt/trusted.gpg are ignored as the file is not readable by user '_apt' executing apt-key.W: http://ftp.de.debian.org/debian/dists/stretch/Release.gpg: The key(s) in the keyring /etc/apt/trusted.gpg are ignored as the file is not readable by user '_apt' executing apt-key.